1. Field of the invention
The invention concerns an ashbox for the hearth of a furnace adapted to burn solid fuel, especially urban waste, discharged onto an upper end of an inclined grate and descending along the grate as it burns to a lower end of the grate, the ashbox comprising an extractor chamber having an upper edge and a concave bottom which is part-cylindrical with horizontal generatrices, the chamber being adapted to be filled with water to a normal level below that of the upper edge, a vertical clinker well under the lower end of the grate, terminating at a lower lip below the upper edge of the chamber and adapted to be immersed in the water in the chamber when filled to the normal level, a scraper blade adapted to bear on the concave bottom of the chamber along a generatrix thereof, means for reciprocating the scraper blade perpendicularly to the generatrices of the concave bottom of the chamber, an inclined wall merging at its lower end with the concave bottom of the chamber along a generatrix thereof, and an overflow lip at the upper end of the inclined wall above the normal level of the water in the chamber, the arrangement being such that reciprocation of the scraper blade pushes ash up the inclined wall and over the overflow lip, into recovery means.
2. Description of the prior art
This conventional arrangement makes it possible to remove the ash in a virtually continuous way without disturbing the conditions under which the fuel is burned, the water seal in the chamber making it possible to isolate the hearth from the external atmosphere so that the burning gases are not deviated from their circulation in the furnace towards the fumes offtake flues if the hearth is at a pressure higher than that of the external atmosphere and so that the admission of combustion-supporting air under the grate is not bypassed by air entering through the ashbox if the hearth is at a pressure lower than that of the external atmosphere.
On falling into the water the hot ash is cooled and in particular incandescent clinker is extinguished and possibly broken up as a result of thermal shock.
The term ash is used generically for any material reaching the downstream end of the grate whether this comprises mineral residues, unburnt materials, incombustible items such as metal parts and, more particularly, clinker or partially vitrified ash aggregates.
Sometimes (and frequently in the case of urban waste) large incombustible objects may be mixed with the solid fuel discharged onto the grate; urban waste commonly contains bicycle frames and component parts of cookers or other domestic appliances; this kind of waste also contains tree stumps where soil trapped between the roots protects the latter from complete combustion so that they form a solid lump.
These foreign bodies may become jammed in the chamber and in any event are not ideally suited to being pushed up the inclined wall; the result is a jam in the chamber.
To clear this is necessary to shut down the hearth, drain the water out of the chamber and then extract the cause of the jam either through inspection hatches in the chamber or along the inclined wall, cutting up the jammed object in situ if necessary.
These operations are difficult, extremely laborious and likely to require the furnace hearth to be shut down for relatively, long periods.
The document FR-A-1 477 877 proposes to provide the lip of the clinker well with a pendulum wall having a counterweight at its lower end so that any large foreign body pushed onto it by the scraper blade can free itself by raising the pendulum wall towards the inclined wall.
This arrangement makes it possible to avoid some jams; on the other hand, there is the risk that the seal provided by the water seal may be inadequate.
What is more, this arrangement cannot prevent jams if the foreign body is jammed between one wall of the chamber and the scraper blade or if the foreign body cannot be removed via the inclined wall. In these cases the previously mentioned problems arise.
The document FR-A-1 477 877 proposes to accelerate the return of the mobile wall by spring means or the like to prevent too much combustion gas and hot ash escaping while the foreign body is freed; this spring increases the risk of jamming, however.
An object of the invention is to enable effective clearance of jams whatever the shape of the foreign body causing the jam with minimum intervention time and minimum disturbance to the operating conditions of the furnace hearth.